1. Field of the Invention
This invention relates to a web of plastic material, more particularly a web of plastic material presenting filtering properties obtained by heat treatment while retaining the base material in such a manner that the base material has undergone a controlled stretch or shrinkage.
2. Description of the Prior Art
Such a web of plastic material as obtained by retaining, for example, a polyester or nonwoven sheet material in a tenter, and heating same, for example, up to 200.degree. C., is known in the art.
This heating process provides stability to the material so that it can be used at a working temperature of up to 200.degree. C. without drastic changes in shape or appearance occurring.
A web of plastic material of this type is relatively expensive, since the base material has to be clamped in a tenter, while heating up to the predetermined temperature also requires very accurate processing conditions.
On the other hand, there is also known a web-shaped plastic nonwoven material obtained by impregnating a plastic nonwoven with plastic resin, and subsequently curing the plastic resin by heat, said treatment being carried out on a roller calender. A drawback of this known web-shaped plastic nonwoven is that the heat treatment must take place within a short time and with high line pressure between the rollers of the roller calender and the base material.
Finally, when subjecting a web-shaped plastic material composed of a laminate of two nonwovens of different plastic materials, the disadvantage arises that, when heating the plastic fibers having the lowest melting point up to the melting temperature, said plastic fibers will fuse together and form a very small mass, all favorable properties of the web-shaped plastic material thus getting lost.